[vnrg] Logical vs. virtual
Didier Colle <didier.colle@intec.UGent.be> Thu, 15 July 2010 13:31 UTC
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Date: Thu, 15 Jul 2010 15:31:48 +0200
From: Didier Colle <didier.colle@intec.UGent.be>
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Subject: [vnrg] Logical vs. virtual
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Dear all, Joe raised the issue of logical vs. virtual. I believe it is worth spending a separate thread on clarifying this point. At the bottom of this email is my proposal. Joe stated: > Note that nothing about these definitions specifies a boundary, i.e., > inside a single machine, etc. I don't think those boundaries are > meaningful in the base Internet anyway. > I.e., IMO, virtual has nothing per se to do with "logical". I.e., a > set of devices on a network that source/sink packets with a single > network address act as a single logical host. That's not 'virtual' to > me, though most 'virtual' things tend to be logical, not all logical > things are virtual. 1) Should I interprete this as logical referring to physical boundaries? 2) This does not define what virtual means or how it differs from logical --> we probably need to formally define virtual inside the vnrg to know what we are speaking about exactly. 3) I agree with "not all logical things are virtual". However, "MOST virtual things TEND to be logical" --> are there any counter examples in which virtual things are not logical? This example also reminded me of the Virtual Router Redundancy Protocol (VRRP): see RFC3768. Not sure what made the authors to call it virtual rather than logical. According to Juniper (sorry for mentioning company names...): http://www.juniper.net/techpubs/software/junos/junos74/swconfig74-routing/html/logical-router-overview2.html > > > Logical Routers and Virtual Routers > > A virtual router is not the same as a logical router. A virtual router > is a type of simplified routing instance that has a single routing > table. A logical router is a partition of a physical router and can > contain multiple routing instances and routing tables. For example, a > logical router can contain multiple virtual router routing instances. > Virtual memory handling in computer systems: http://en.wikipedia.org/wiki/Virtual_memory#Segmented_virtual_memory > > > Page tables > > Almost all implementations use page tables > <http://en.wikipedia.org/wiki/Page_table> to translate the virtual > addresses seen by the application program into physical addresses > <http://en.wikipedia.org/wiki/Physical_address> (also referred to as > "real addresses") used by the hardware to process instructions. Each > entry in the page table contains a mapping for a virtual page to > either the real memory address at which the page is stored, or an > indicator that the page is currently held in a disk file. (Although > most do, some systems may not support use of a disk file for virtual > memory.) > > Systems can have one page table for the whole system or a separate > page table for each application. If there is only one, different > applications which are running at the same time > <http://en.wikipedia.org/wiki/Multiprogramming> share a single virtual > address space, i.e. they use different parts of a single range of > virtual addresses. Systems which use multiple page tables provide > multiple virtual address spaces—concurrent applications think they are > using the same range of virtual addresses, but their separate page > tables redirect to different real addresses. > > > [edit > <http://en.wikipedia.org/w/index.php?title=Virtual_memory&action=edit§ion=4>] > > Conclusion: not really clear definition of both terms. My proposal: * Logical is opposite of physical: logical always requires mapping to physical. Of course, logical can be mapped on some other logical thing that itself needs mapping to physical. More formally, this mapping is a general function: 1) nothing should prohibit mapping multiple logical names/addresses to a single physical address/thing, 2) not all physical entities must have a logical counterpart. Also the source and target of the mapping should not be restricted in terms of types: names can be mapped onto other names or names can be mapped onto address for examples. * Virtual is a specialized form of logical, as the virtualization technology still needs to map a virtual entity onto a single non-virtualized (physical or isolated uniquely identifiable logical) entity, while guaranteeing isolation. More formally, this mapping is an injection of tupples of <entity, virtual instance ID> onto entities of the same type (thus not mapping of names into addresses for example: virtualization of name space, means mapping <virtual name, instance ID> on to unique identifiable <name>, virtualization of virtual memory (I would rather call this logical memory) page tables <virtual page number (seen by application), instance ID> on to <system wide page number>). Kind regards, Didier -- Didier Colle Ghent University - IMEC - IBBT Department of Information Technology (INTEC) Gaston Crommenlaan 8 bus 201, B-9050 Gent (Ledeberg) Email: didier.colle@intec.UGent.be MSN: didiercolle@hotmail.com Skype: didiercolle Tel. +32 9 331 4970 Fax. +32 9 331 4899 Mobile: +32 473 295655 WWW: www.ibcn.intec.UGent.be
- Re: [vnrg] Logical vs. virtual Didier Colle
- Re: [vnrg] Logical vs. virtual Sangjin Jeong
- [vnrg] Logical vs. virtual Didier Colle
- Re: [vnrg] Logical vs. virtual Joe Touch
- Re: [vnrg] Logical vs. virtual Joe Touch
- Re: [vnrg] Logical vs. virtual Sunay Tripathi
- Re: [vnrg] Logical vs. virtual Martin Stiemerling
- Re: [vnrg] Logical vs. virtual $witch
- Re: [vnrg] Logical vs. virtual Martin Stiemerling
- Re: [vnrg] Logical vs. virtual $witch
- Re: [vnrg] Logical vs. virtual Roland Bless
- Re: [vnrg] Logical vs. virtual Roland Bless